1. Field of Invention
This invention relates generally to circuits for recharging Nickel-Cadmium (Ni-Cad) batteries, and more particularly to a piggyback battery conditioner unit usable in conjunction with a standard battery charger and a rechargeable Ni-Cad battery pack which normally is adapted to be seated on and connected to the charger, the unit when in use being physically and electrically interposed between the charger and the pack and serving to so condition the pack whereby when the pack is recharged it regains its full rated energy capacity.
2. Status of Prior Art
The major alkaline secondary or rechargeable battery is the Nickel-Cadmium (Ni-Cad) battery. It is noted for its high power capability and long cycle life, as well as for its ruggedness and reliability. Ni-Cad battery power packs, which are constituted by an encased set of serially-connected Ni-Cad batteries are widely used as power supplies for portable electronic equipment such as battery-powered radio and TV sets and hand-held camcorders.
A problem frequently encountered with Ni-Cad battery power packs is the degradation of the batteries resulting from repeated recharging operations. It has been demonstrated that as a result of a "memory effect" or "fading," that the energy capacity of a Ni-Cad battery can be decreased as much as 60 to 80% under operating conditions. Thus a user of a battery-operated camcorder often finds that while at the time the camcorder was first purchased, the Ni-Cad power pack was then capable of powering the camcorder for more than half an hour of operating time before it became necessary to recharge the power pack, after the power pack had been recharged several times, the power pack then becomes incapable of supplying power to the camcorder for more than fifteen minutes of operating time.
This characteristic of a Ni-Cad battery power pack represents a serious practical drawback in that the power pack does not live up to its rated energy capacity, and the portable electronic device which depends on the power supplied by the battery pack then ceases to function. The charger for the power pack, which includes a rectifier coupled by a step-down transformer to an AC power line to provide a DC output is not portable, for the charger must be plugged into an AC power line. Hence the user, say, of a camcorder powered by a Ni-Cad battery power pack, who goes on a field trip with the expectation of being able to record for a full half hour of operating time may find that after 10 minutes of operation the camcorder can no longer record. And the user of the camcorder is not then in a position to recharge his power pack.
Yet the reason for the failure of the power pack to live up to its rated energy capacity is not due to any defect in the particular Ni-Cad cells included in the pack, for all Ni-Cad batteries will fall short of their rated energy capacity unless they are properly conditioned before being recharged.
The "memory effect" or fading phenomenon characteristic of all Ni-Cad batteries is induced by shallow cycles of discharging and recharging. For example, if a Ni-Cad battery is operated so that it repeatedly delivers 50% of its full rated energy capacity and is then recharged, this battery will exhibit a sharp decrease in its ability to deliver a proper battery voltage when the battery is being operated beyond 50% of its full capacity. When a Ni-Cad cell is cycled repeatedly but is not fully discharged (shallow discharge), the battery gradually deteriorates to a much lower capacity level.
Manufacturers of electronic equipment powered by Ni-Cad battery power packs are fully aware of the "memory effect" phenomenon. Hence in the instruction manuals which accompany the equipment, the user is instructed that when the power has fallen to a degree at which the equipment no longer operates properly, that the battery pack first be fully discharged before it is placed in the charger for recharging.
What this means in practical terms, say, with a hand-held vacuum cleaner that includes a Ni-Cad battery pack, is that the user before seating the vacuum cleaner on the AC-powered charger unit, must continue to press the operating button of the vacuum cleaner to effect further discharge of the battery power pack.
Despite the fact that the user has read the instruction manual and is aware of the need for deep discharge before recharging, he will often not bother to carry out this operation, for it requires, even though his equipment is no longer functioning properly, that he continue to press the operating button. And when doing so, the user does not know when a state of deep discharge is attained, for the typical Ni-Cad battery pack has no such indicator.
And for each conscientious user of the equipment who has read the instruction manual, there are many who have not--hence the typical user makes it a practice to recharge his battery pack as soon as he senses that he is not getting adequate power. This practice inevitably leads to a degradation of the batteries because of the memory effect.
Indeed, Ni-Cad rechargeable batteries are thought by many consumers to be inferior to non-rechargeable long-life alkaline batteries; for while Ni-Cad batteries work well for a fairly short period after their purchase, the more often they are recharged, the poorer is their energy capacity.
The prior art recognizes that the "memory effect" exhibited by Ni-Cad batteries can be eliminated by reconditioning the batteries; that is, by subjecting them to deep discharge before recharging. One reference in the prior art provides a discharge circuit for Ni-Cad batteries which reduces its state of charge to a nominal zero, after which the battery can then be recharged to restore the battery to its full rated energy capacity.
A Ni-Cad battery, when fully charged, has a nominal voltage of 1.2 volts per cell, and it is regarded as "empty" when its voltage falls to 1.1 volts. A battery power pack consisting of 30 Ni-Cad cells in series has a nominal voltage of 36 volts, and when the voltage drops to 33 volts, the pack is depleted.
In the Brown patent, the discharging circuit includes means to sense the voltage across the battery terminals, and when the voltage sensed indicates that the battery is depleted, a switch is activated to connect a resistor across the terminals of the battery to deep discharge the battery.
The Yefsky U.S. Pat. No. 4,302,714 discloses a battery charger circuit for deep discharging a Ni-Cad battery to a predetermined level above cell reversal and then recharging the battery to its rated capacity. Similarly, in the Kapustka U.S. Pat. No. 4,084,124, a conditioner circuit acts to first deep discharge a Ni-Cad battery before it is recharged.
The Munning Schmidt et al. U.S. Pat. No. 4,849,681 discloses a discharger for a Ni-Cad battery to effect deep discharge before recharging takes place. In the Gabor et al. U.S. Pat. No. 4,878,007, the charger for a Ni-Cad battery produces repetitive periods of DC charging and discharging.
Of background interest is the Williams, U.S. Pat. No. 4,670,703, which points out that Ni-Cad batteries, should they be overcharged, such overcharging may result in internal damage to the battery or even explosion. When a Ni-Cad battery is already fully charged, further charging leads to a temperature rise causing "gassing." Since the battery is usually hermetically sealed, the internal pressure produced by gassing could explode the battery. In this patent, a microprocessor controls the charging rate to effect a safe charging level.
In the Webb U.S. Pat. No., 3,413,534, the battery charger for a Ni-Cad battery is microprocessor-controlled to obtain a discharge mode of operation and an auto cycle charging operation which is carried out after a discharge operation.
The disadvantage of prior art battery dischargers and chargers for Ni-Cad batteries is that they do not lend themselves to ready use with Ni-Cad battery power packs which are designed to be seated on and connected to a standard charger.
For example, a typical, commercially-available camcorder, such as a Sony portable camcorder, is provided with a Ni-Cad battery power pack whose output terminals are connected to the camcorder when the power pack is plugged into the camcorder. When it is necessary to recharge the pack, the pack is then unplugged from the camcorder and seated on a battery charger operated from the A-C power line, the pack terminals then engaging the terminals of the charger. When the pack is fully charged, the pack can then again be plugged into the camcorder. The problem with the Sony arrangement is that should the power pack not be deep discharged before being recharged, it will not upon recharging regain its full rated capacity.
If, therefore, one wished to deep discharge the Ni-Cad battery power pack with a discharge circuit of the type shown, say, in the Brown patent, one would have to connect the terminals of the power pack to a cable connecting the pack to the Brown discharge circuit, and upon completion of the discharge, to disconnect the pack from the cable and then seat the pack on the Sony battery charger.
While this procedure would present little difficulty to a skilled technician, the typical camcorder user possesses little in the way of technical skills. The need to cable-connect a battery pack to a special discharge circuit and then disconnect the cable each time it became necessary to recharge the pack renders the Brown arrangement unsuitable for the typical user.
One must bear in mind that literally millions of electronic devices powered by Ni-Cad battery packs are currently in use, and each user is provided with a standard charger adapted to recharge the power pack. The need therefore exists for a self-sufficient Ni-Cad battery conditioner unit that would make it possible for the typical user to deep discharge the power pack, yet in no way require modification of the pack or of the charger therefor.